1. Field of the Disclosure
The present disclosure relates to a fuel cell and a generation system and method using the same, and particularly, to a fuel cell using biogas as a fuel, and a generation system and method using the same.
2. Background of the Disclosure
A fuel cell is an apparatus which electrochemically reacts fuel with an oxidant to generate electrical energy. Unlike general cells, the fuel cell consumes fuel to generate power.
Examples of the fuel cell include various kinds of fuel cells such as a molten carbonate fuel cell (MCFC), a polymer electrolyte membrane fuel cell (PEMFC), a solid oxide fuel cell (SOFC), a direct methanol fuel cell (DMFC), a direct ethanol fuel cell (DEFC), and a phosphoric acid fuel cell (PAFC).
Among the fuel cells, the MCFC is a fuel cell which is the most commercialized. In this specification below, the MCFC will be described unless otherwise specifically mentioned, but it is not intended that the scope of the present invention are limited thereto.
To provide a detailed description on the MCFC, the MCFC is a fuel cell that uses hydrocarbon as a fuel. Generally, the MCFC includes a fuel electrode (anode), an air electrode (cathode), and a matrix, and an electrolyte is impregnated in each of the elements. A hydrogen fuel gas generated through a reforming reaction of natural gas is injected into the fuel electrode of the MCFC, and carbon dioxide and oxygen are supplied to the air electrode, whereby a carbonate ion (CO32−) is generated in the air electrode. The carbonate ion generated in the air electrode moves from the air electrode to the fuel electrode through the electrolyte of the matrix which is disposed between the fuel electrode and the air electrode, and an electron generated in the fuel electrode passes through an external circuit. In this case, the electrolyte is normally in a solid state, and when the fuel cell is steadily operated, the temperature rises to about 650° C. to liquefy the electrolyte.
Generally, when fuel cells having a high operating temperature like the MCFC, a perovskite catalyst and a noble metal catalyst such as Pd and Pt, which show a high combustion reaction activity are used. This refers to a catalytic combustion, and is a method of performing flameless combustion on the surface of a solid catalyst by the oxidation-promotion action of the catalyst by supplying a mixed gas of a fuel and air to a combustion apparatus including the solid catalyst therein. The catalytic combustion has been used in the wide fields for the purpose of completely oxidizing fuel or hazardous substances into oxygen to obtain energy or make them harmless, but there is a problem in that the service life thereof is reduced by effects caused by catalyst poisoning when sulfur (S) is supplied.
Meanwhile, as the fuel supplied to the fuel cell, natural gas is mostly used, but in an effort to diversify fuel sources, fuel cells, which use biogas including anaerobic digestion gas (ADG) which includes, as a main component, methane (CH4) generated as food waste, livestock manure, and sludge from sewage treatment plants are digested in a digestion tank, or landfill gas generated from waste landfill sites as a renewable gas in addition to natural gas, have also emerged.
Hereinafter, fuel cells which use anaerobic digestion gas as a renewable gas will be described in the present specification, but it is not intended that the scope of the present invention is limited thereto, and it is needless to say the present invention may also be applied to a fuel cell which uses various biogases.
The anaerobic digestion is a traditional technology which has been established and developed for a long period of time, and has been used in treatment of liquid and semi-solid waste in regions where the climate is relatively mild. The anaerobic digestion has been applied to waste having a solid content of 10% or less in most of the process, and has also been tried to be applied to solid waste having a solid content of about 25% by recent development in technology. The anaerobic digestion is also referred to as methane fermentation, and has been applied for the main purpose of treating wastewater or waste and simultaneously collecting energy which is methane. The methane fermentation has not been actively used since the spread of the activated sludge method, but has been used as an energy means for actively replacing petroleum for the purpose of collecting energy from large amounts of organic waste in the urban garbage as well as manure and sewage due to the advantage in that collected gas (CH4: 50 to 70%, CO2: 30 to 50%) has enabled to be used as a fuel since the oil shock in the mid-1970s, and the power consumption is low. In this regard, most of the sludge treatment systems from sewage treatment plants in Korea are composed of an enrichment tank→a digestion tank→sludge→a dewatering apparatus to install the digestion tank, and the aforementioned anaerobic digestion reaction occurs in the sewage sludge to be introduced into the digestion tank to produce ADG which includes methane as a main component.
ADG includes impurities such as sulfur components such as hydrogen sulfide (H2S), carbonyl oxysulfide (COS), and methyl mercaptan (CH3SH) and siloxane, and a gas purification process for removing impurities contained in the gas is required for the ADG to be used as a fuel for a fuel cell. The gas purification process as described above is composed of a desulfurization process for removing H2S and a siloxane removal process, and the ADG is subjected to the ADG gas purification process, and then is supplied as a fuel for a fuel cell.
Even though the ADG purified by being subjected to the gas purification process in the related art is supplied as a fuel for a fuel cell, hydrogen sulfide components are contained in an amount of 5 ppm in the ADG, and thus affect a decrease in voltage of the cell and causes deterioration in performance of the cell due to poisoning effects of a fuel cell reforming catalyst (electrolyte). Furthermore, when a room-temperature desulfurization process is additionally applied in order to maintain the concentrations of the sulfur components within a permissible concentration (0.17 ppm or les), there is a problem in that the economic efficiency is decreased due to costs of a desulfurization apparatus and costs of replenishing a desulfurizing agent.
Further, since methane components and carbon dioxide components are included at a level of 50 to 70% and 30 to 50%, respectively in the ADG, there is a problem in that the voltage of a fuel cell drops due to an increase in partial pressure by carbon dioxide when the carbon dioxide components are supplied to the fuel electrode of the fuel cell.
Therefore, there is a desperate need for a technology required for solving the problem when biogas is used as a fuel for a fuel cell.